This invention relates to steels and to a multiphase microalloyed steel having particular utility in long product (e.g., bar, rod, and wire) applications.
Forging is a commercially important method of producing finished or semi-finished steel products, wherein a piece of steel is deformed in compression into desired shapes. Forging may be accomplished with a wide range of processes. The steel may be heated to and forged at a high temperature, or forging may be accomplished at ambient temperature. The steel may be deformed continuously or with repeated blows. The steel may be formed without a die, or in a closed die to obtain closer tolerances of the final part. Steel forgings range in size from less than one pound to many tons in size, and hundreds of thousands of tons of steel are forged each year.
Until the 1970s, the vast majority of cold-forged and hot-forged steel forgings were made using "plain carbon" or low alloy steels with a carbon content selected to yield a combination of forgability and final properties. High strength forgings usually contain medium carbon contents of about 0.2-0.5 weight percent. This carbon content is required to permit the forging to be heat treated to the required strength through a post-forging heat treatment. While the moderately high carbon content is beneficial from the standpoint of achieving high strengths in the heat-treated condition, it also results in cold ductility and toughness that are insufficient for many requirements. Therefore, when these steels are to be supplied in cold forging applications, they must be subjected to a spheroidizing anneal prior to the cold deformation. Hence, until the early 1970s, the steels available for these high strength, hot and cold forging applications were medium carbon steels which could be heat treated to adequate strength levels at a very high cost of production, which included the spheroidizing anneal and stress relieving treatments.
In the early 1970s, attempts were made to reduce the cost of producing high strength hot forgings through the use of medium carbon microalloyed steels. Since these steels develop precipitation hardened ferrite-pearlite structures in the as-forged condition, they can achieve yield strengths of 85-90,000 pounds per square inch without the need for post-forging heat treatments. Unfortunately, these ferrite-pearlite steels exhibit low ductility and toughness and therefore are not usable in cold forging or applications requiring acceptable toughness such as safety-related items including striker bolts, steering knuckles, and center links in automobiles, and fasteners and other non-automotive applications.
End users' concerns for stronger, tougher, and more cost effective steels cannot be satisfied by either the quench and temper steels because they are too expensive, or the ferrite-pearlite steels because they have insufficient properties. Although medium carbon microalloyed steels are now used in some forgings, there remains the problem of insufficient strength and toughness in the forged components, particularly in safety-related applications. A new alloy design is required for optimization of performance and cost in particular kinds of applications. The present invention fulfills this need, and further provides related advantages.